A brake system of this type is disclosed in Japanese patent application No. 94-171487. The brake system concerned is an anti-lock brake system which, in contrast to conventional brake systems operating according to the recirculation principle, includes neither a low-pressure accumulator nor outlet valves in the return lines from the wheel brakes to the suction side of the return pump. To produce a dynamic pressure in the wheel brakes, the return lines have only restrictors which produce a pressure gradient between the wheel brakes and the suction side of the return pump. The pressure in the wheel brakes is reduced by closing the inlet valves in the brake branch lines to the wheel brakes so that the pump pressure will not reach the wheel brakes. The braking pressure is decreased because the pressure fluid is permanently discharged from the wheel brakes.
An object of the present invention is to retrofit a brake system of this type for traction slip control by minimum possible additional effort, i.e., the least number of electric lines the minimal additional logic structure for valves.
Thus, the present invention permits a brake system in vehicles with one driven axle to perform traction slip control operations by at most four additional solenoid valves. Only the driven wheels require an outlet valve which permits a sufficient braking pressure increase by the return pump alone. There is no need for a low-pressure accumulator in traction slip control. When three individual two-way/two-position directional control valves are chosen, alternatively, either the change-over valve in the suction line to the return pump or the separating valve in the brake line, or both these valves can be operated hydraulically. Solenoid valves are preferably used as outlet valves in order that an amount of braking pressure can be increased or decreased in the wheel brakes in conformity with requirements. When the change-over valve and the separating valve are united in a combination valve, the combination valve can either be actuated electromagnetically and, thus, necessitate the second actuation logic beside the outlet valve, or it can be configured as a hydraulically operated combination valve, thus obviating the need for further actuation and electric lines.